Positron lifetimes and phase transitions in a cholesteric liquid crystal

Positron annihilation lifetimes have been studied as a function of temperature in the liquid crystal cholesteryl oleyl carbonate (COC). The delayed component (tau3) arising from the pick-off annihilation of orthopositronium shows marked changes as the sample goes from the isotropic liquid phase to the glassy solid phase. Two shorter lived components tau2 and tau1 are also seen in the lifetime spectra; the former (tau2) disappearing below 190 K. These two lifetime components do not show any appreciable change with the phase transitions taking place in the system while tau3 is very sensitive to many of these transitions. A discussion of the correlation of the observed changes in lifetimes with the changes in the molecular orientational order and the intermolecular interactions is presented. To elucidate further the effect of orientational order on the pick-off lifetime, we have presented the data on an oriented glassy solid phase of COC. (author)